“Who decided that 20 percent was the new standard tip?” That was a question I heard on the radio awhile back while fighting traffic to get to work. As a restaurant worker for the better part of the last 15 years, I feel qualified to answer this question, as well as the larger question of, “Why do we tip at all?”

And then there is the follow-up question: “Is it servers or restaurants or customers who decide the standard tip?” My answer is simple: “Yes.” There are multiple reasons for the increase in the standard tip percentage. The two most important are the increase in the cost of living and the increased expectations of service.

The cost of living has increased nearly tenfold since 1950. The cost of a meal at a casual-dining restaurant, however, has only increased by sevenfold in that time. That means that for a server to have the same standard of living, the tip percentage has to have increased about 40 percent from what it was in 1950. Since the standard tip in 1950 was 10 percent, that means that it needs to have increased by about 4 percent to provide the same standard of living.

If this were the only factor, then a standard tip of 15 percent would be just dandy. However, this is not the only factor that has increased the standard tip percentage.

In addition to the change in the relationship between the cost of living and the cost of a meal, the expectations customers have for their servers have changed. People tend to forget that restaurants are selling two products: food and service. In 1950, it was not uncommon for a server to have a section of eight or 10 tables. Service was prompt and efficient, and as long the meal came out quickly and correctly, everyone was happy. This means that in 1950, each table was paying for 10 percent of a server’s time. In 2010, however, customers expect much more.

Servers are now expected to refill drinks before they are half-empty, clear dishes as soon as they are used, serve multiple courses, check back frequently, and provide pleasant small talk. All of these things mean that servers have less time for other tables. As a result, restaurants have scaled back the size of a server’s section to three or four tables. That means that customers today are purchasing 20 percent of a server’s time. Since each server only has so much time in an hour, and the demand for that time has increased, the price paid must increase, too — in this case, in the form of an increased tip. If a 10 percent tip purchased 10 percent of a server’s time in 1950, then in order to purchase 20 percent, the tip must increase to 20 percent as well.

Actually, when you factor in the increase in the cost of living versus the cost of a meal, servers should be making nearly 30 percent.

“Why tip at all?” you may ask. This, too, I can answer: to keep the cost of a meal down. Paradoxically, by tipping servers, customers help lower the cost of a meal. As I mentioned before, restaurants sell food and service. The price on the menu covers the cost of the food (as well as other costs, like the facility, kitchen staff, power, etc.); the tip covers the cost of service.

So, the next time you complain about the expectation of a 20 percent tip, remember that you are paying less for your server’s time than your grandfather did, and by doing so you are keeping the cost of your meal down as well.

David L. Holt of Denver has spent nearly 15 years in the restaurant industry in various capacities.